1. Field of the Invention
The present invention relates generally to methods for calculating the physical parameters for conduit for use in a conduit expansion process and, in particular, to a method for determining the pressure capability of such a conduit, including the hydrostatic design basis for expanded conduit, preferably conduit manufactured from polyvinyl chloride (PVC).
2. Description of the Related Art
Open-cut installation of polyvinyl chloride (PVC) conduit for new potable water systems has been a proven methodology for many years. Creating PVC conduit having improved hydrostatic stress rating through expansion and orientation of the polymer (PVCO) is also an established technology. Sliplining (or placing a secondary pipe within a host pipe) in existing pipelines with butt-fused sections of polyethylene (PE) pipe is also a familiar technology.
Pipe made from PVC has been a major success story for the plastics industry for at least three decades. This product is widely used for potable water and wastewater in piping systems ranging in sizes from ½-inch nominal diameter to more than 30 inches diameter. Such piping systems are normally joined with bell-and-spigot gasketed or solvent-cement joints. There are ASTM International (ASTM) Standard Specifications (Annual Book of ASTM Standards 2003, Vol. 8.04, Plastic Pipe and Building Products, ASTM International, West Conshohocken, Pa. 19428) and American Water Works Association (AWWA) Specifications (AWWA Standards, American Water Works Association, Denver, Colo. 80235) that cover these products, including the tests and calculations needed to assign a pressure rating to the finished pipe product.
Unlike polyethylene pipe, which is generally joined by heat fusion techniques, PVC pipe has until now been limited to applications that can be serviced with bell-and-spigot joints. This limitation has restricted use of PVC pipe in sliplining rehabilitation applications. Sliplining is an effective method of pipeline rehabilitation where pipe sections with diameter slightly smaller than the deteriorated existing pipeline are sequentially butt-fused and inserted into the existing pipeline. Lack of formulations and techniques for achieving butt-fused PVC joints with strength approaching that of the PVC material itself has been the primary barrier to using PVC in these applications.
Successful rehabilitation of damaged pipelines requires some means of achieving a close fit between the original pipeline and the liner pipe. This can be achieved by swaging polyethylene pipe to temporarily achieve a slight reduction in diameter, which memory-driven expansion later eliminates. In other applications, an annular space is left between the pipes, and is then filled with grout. In all sliplining applications, reduction of the pipeline capacity by reducing diameter is a major concern and limitation. Inserting PVC pipe, which is inherently stronger than PE pipe at the same thickness, and then expanding the inserted pipe to match its outside diameter to the inside diameter of the original pipeline, minimizes this capacity reduction. Quantifying the strength increase achieved in any given pipe expansion, and capturing this increase in a pressure rating for the rehabilitated pipeline, is beneficial.
Presently, there is no data or methodology to determine the Hydrostatic Design Basis (HDB) of expanded oriented pipe other than at specific predetermined uniform expansion. At zero expansion, the HDB of the PVC conduit is known to be 4000 psi. At two times expansion, the HDB of the PVC conduit is 7100 psi. However, there is a need for a methodology for definitively determining the HDB of expanded PVC conduit at any value between these values. Further, as a result of field conditions, the variance in wall thickness of the PVC starting stock, when expanded, reduces the data confidence level to less than the required 85% when conventional technology is used. Accordingly, there is a need for a methodology that yields results within the required confidence level and fill in the knowledge base between known points.